How To Replace The Gas Valve Assembly On A Furnace

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The gas valve assembly on a furnace provides the vital function of controlling the inflow of gas based on your heating needs. If you're experiencing interruptions in heating and you have checked out the fuses and pilot light and found no problems, you might need to replace the gas valve assembly.

Replacement isn't difficult but is best left to those with a bit of heater repair experience. If you feel unsure about the job at any point, set down your tools and call in a professional technician like Bristol Heating & Air Conditioning. Note that this is a general guide and doesn't replace using your owner's manual as the specific instructions for your unit might be different.

Things You Need:

Step 1: Access the Gas Valve

Turn off the power to your unit at the breaker or the shut-off switch next to the unit. Make sure the gas shut-off valve is also turned off.

Remove the upper panel on the back of the furnace. The panel is typically removed either by sliding the panel upwards or twisting a set of knobs on the panel and then pulling it free from the furnace. Set the panel aside somewhere safe.

The gas valve is the control box-looking rectangle near the center bottom inside the furnace. Consult your owner's manual if you're unsure which part is your valve assembly.

Step 2: Unhook Old Gas Valve

Locate a junction of two pipes off to one side of the gas assembly. These are the gas supply pipes, and they are joined together with a union.

Hold one pipe wrench firmly on the section of pipe coming in through the side of the heater. Use another wrench to twist the union until it comes loose. Pull off the union and set it aside.

Leave the pipe coming in through the side alone and concentrate on the pipe that was on the other side of the union. This gas inlet pipe is now hanging out of the gas valve. Use a wrench to twist this pipe off of the valve.

Use a digital camera or camera phone to take several zoomed in images of the gas valve and its wiring. You want a picture of every point where wires connect making sure you focus on any potential wire color layouts.

Once you are satisfied you have clear informative pictures, unhook all of the wires that connect the valve to the furnace. At this point the valve should still be attached to the manifold.

Use a screwdriver to remove the screws attaching the manifold to the furnace. Place the screws in your pocket for safekeeping. Set the manifold and valve assembly on the floor for stability. Use a wrench to unscrew the gas valve assembly form the manifold. The valve is threaded onto a grooved end on the manifold so it might take several firm twists to separate the two.  

Step 3: Replace the Gas Valve

Remove the new gas valve from the packaging materials, and take off any caps or plugs blocking the holes. Apply a pipe sealant to the threaded end of the manifold, then manually twist the appropriate end of the valve onto that manifold end. Once you can no longer manually twist the valve, use a wrench to finish up tightening.

Reattach the manifold using the screws, then hook all of the wiring back up using your pictures as a guide. Apply pipe sealant to the end of the gas valve pipe, and thread it onto the open end of the gas valve. Twist the union back onto the exposed end of the valve pipe, then attach to the pipe coming in through the side of the furnace. Use wrenches to tighten.

Replace the rear panel of the furnace. Turn on the power and gas supply, then take your furnace for a test run to see if everything is installed correctly.